The pressure-volume behavior of the lung is determined by the surface-tension at the liquid air interface and by the stress-stain characteristic of the alveolar wall. The structural proteins, elastin and collagen, have been considered to be the major biochemical determinants of the mechanical properties of the lung. The wall has been considered to be passive in nature with changes of elasticity requiring alterations of the structural proteins. Our studies of parenchymal strips indicate that the alveolar wall has an active contractile component which is independent of airway and vascular smooth muscle. This contractile capability is related to non-muscle cells which contain actin filaments which we have demonstrated by electron and immunofluorescent microscopy. The major hypothesis of this proposal is that the alveolar septa is a contractile unit and that this property can influence the function of the lung. We will test this hypothesis by determining the active and passive stress-stain characteristic of isolated alveolar septae. These measures will be correlated with the morphometric determination of the types and numbers and sizes of cells within the septa. The tissue will be characterized further by immunofluorescent techniques to localize actin, myosin, and collagen type. In addition, direct measures of actin, myosin, collagen and elastin content will be accomplished using new microassays. The isolated alveolar septa preparation provides a method to study parenchymal function without the confounding influences of airway or vascular smooth muscle. This will allow us to directly relate the mechanical properties of the septa to its structure and composition. This analysis will be completed in normal and fibrotic tissue from rats.